Oil dropped more than $6 yesterday thanks to mounting concern about the U.S. economy, Reuters reported.

The one-day drop of $6.44 is the largest since 1991, when prices lowered at the beginning of Operation Desert Storm, according to the news agency. U.S. crude reportedly settled at $138.75. 

Investors have been apparently worried about the demand for oil, with consumers using less gasoline due to high prices and a worsening economy.